A visual scene can be composed of objects of different type such as natural video, two-dimensional, three-dimensional synthetic images, graphics or text.
A video communication system, like for example a mobile telecommunication system, typically comprises a first mobile phone and a second mobile phone, including an encoder and a decoder, respectively. In order to transmit a visual scene from the first mobile phone to the second mobile phone, the different objects are encoded by the first mobile phone and transmitted independently to the second mobile phone. At the decoder side the scene is recomposed by decoding the different objects and then by rendering the decoded objects. The rendering usually comprises a geometric transformation step able to map the object on the scene and a step of blending the mapped objects together, resulting in a final scene.
An object typically comprises chrominance U and V, and luminance Y components forming the color of the object. During the blending step of a current object N with an object M, a transparency value Nalpha can be associated to the object N, said transparency value influencing the chrominance and luminance values Pyuv of a blending object P such that:Pyuv=[(255−Nalpha)*Myuv+Nalpha—y*Nyuv]/255
The transparency of a pixel of a blended object is obtained according to the shape of said pixel. The transparency permits to determine which object is behind or in front of another object in the scene. The shape is associated with a pixel of a frame of a scene. It permits to know if a pixel is outside or inside an object. If it is outside, the shape has a null value, otherwise it has a value equal to 1.
For a pixel having coordinates (x,y), the shape value 0 or 1 corresponding to these coordinates is taken and attributed to the luminance of this pixel. In the case where the luminance values the crosses), the shape used for the chrominance value U or V(x,y) is the shape of the pixel of coordinates (2x−1, 2y−1), i.e. the top right pixel of a square of 2×2 pixels associated with the chrominance value.
One inconvenient of this solution is that the chrominance of the blended object is false. This results in a bad visual quality for objects having an arbitrary shape. This observation is illustrated in the example of an object constituted of the letter “e” and represented by stripes in FIG. 2a to 2c. FIG. 2a is the original object, FIG. 2c is the result of the composition and FIG. 2b explains how the result is obtained. Each square represents a pixel. If one takes the pixel whose coordinates are (4,2), the pixel whose shape will be used for the chrominance component is the pixel (3,1) as shown in FIG. 2b. The value of this shape is 0, that is to say it is outside the letter “e”, and the chrominance value will not be taken into account, as shown in FIG. 2c where the pixel value is represented by dots. The result is false because the chrominance value should have been taken into account and the result should have been the same as in FIG. 2a. 